In conventional inkjet printers utilizing jetting of liquid ink, proper temperature control may be required in order to maintain a uniform printing because the viscosity of the liquid ink in the inkjet head chip changes according to the ambient and/or head chip temperatures. The temperature related viscosity change affects the drop volume of the ejected ink, and as a result, affects the quality of an image printed on a printing medium, such as, e.g., on a sheet of a paper. For example, when temperature increases, the viscosity of ink tends to decrease, and thus the ejection amount according to each nozzle may increase. As a result, the resulting output image may have a higher optical density. On the other hand, when temperature decreases, the viscosity of ink tends to increase, and thus, the ejection amount according to each nozzle may decrease, resulting in the output image having a lower optical density.
When printouts are repeatedly outputted in a high speed/high resolution mode, the temperature of a head chip may gradually increase, and when the temperature exceeds certain level, a stable ink ejection cannot be expected, and it may thus become necessary to stop the printing operation for certain cooling time.
Moreover, when an array of head chips are employed, e.g., in a wide array printer, temperatures between adjacent head chips may differ, and such different temperatures may result in an inferior image quality. Accordingly, the controlling of temperature may be of a greater concern, e.g., for a wide array printer than it would be e.g., for a shuttle type printer.
Generally speaking, synchronization of the head chip(s), the CPU and other mechanical device(s), e.g., the motor is based on the system clock frequency. Information with respect to the location on a printing medium on which to eject ink is converted to a signal assigning the nozzle location of each head chip and a load signal according to timing requirement.
For example, FIG. 1 illustrates control signal synchronization based on the system clock frequency SCLK for one nozzle. Each nozzle generates ink bubbles when a current flows through a heater. The process is controlled by the on/off time of a Fire-pulse signal. The Fire-pulse signal shown in FIG. 1 includes a T_warm time, a T_fire time, and a T_No fire time. During the T_warm time, the head chip is preheated to a predetermined temperature before the supply of the bubble generation amount of current to the heater. A warming up signal during the T_warm time is called a Fire_start signal. The Fire_start signal increases the temperature of a chip without generating the bubbles, and thus a current lower than the current for ejecting bubbles of the ink is supplied during the T_warm time. During the T_fire time, the bubbles are generated in the heater and drops of the ink are ejected. The Fire_On signal is the current signal for generating the bubbles, and the Fire_On_Add signal is a signal to compensate for a margin of ejection point of time between chips and or between nozzles. Accordingly, the T_fire time, i.e., the total time of supplying the current to the heater, is obtained by adding the times in which the Fire_On signal and the Fire_On_Add signal are turned on. The T_No fire time is not a time during which an electric physical signal is applied to the heater, but a time obtained by adding the refill time necessary for re-supplying the ink, and the nozzle meniscus stabilizing time. As shown in FIG. 1, even when the ejection amount of the ink may be affected by the temperature, the control signals applied to the head chips do not take the temperature into account.
Briefly, the control algorithm based on the temperature includes the following. When a print request signal is received, a determination as to whether the head chip is overheated. When the head chip is overheated beyond certain threshold temperature, the printer halts the operation, and when the head chip cools down below the threshold temperature, the printer resumes operating. The image quality may also deteriorate when the viscosity of ink increases, or when the temperature of the head chip becomes too low, and thereby, reducing the amount of the ink ejected. Accordingly, to raise the temperature of the head chip is increased by applying a pulse for increasing the temperature of the heater or by ejecting the ink. The above is an algorithms generally employed for a shuttle printer, and allows the printer to operate only within a predetermined window of operating temperature range, and halts the operation of the printer outside the temperature range.
In a wide array printer, each head chip is assigned to a portion of a row of an output pattern, unlike a shuttle printer, in which one head chip outputs all of the pattern by shuttling across the row. Thus, in a wide array printer depending on the particular pattern, it is possible that only specific ones of the head chips may be heated.
For example, in FIG. 2, example pattern outputs (a) and (b) illustrate the performance difference between one page to the next. Referring to FIG. 2(a), the image pattern calls for only specific head chips A, C and E to eject ink, thus, increasing the temperatures of only the head chips A, C and E. FIG. 2(b) illustrates an effect of the increased temperature of the head chips A, C and E on the output image of the next page. Even when the entire image having a uniform light color is intended, the optical density of image output by the heated head chips A, C and E is different (i.e., darker) from the optical density of the image output from head chips B, D and F. In other words, the head chips A, C and E have a higher optical density than the head chips B, D, and F due to the drop volume increase. When there is an optical density difference between adjacent head chips despite of the same input, lines appear in the image between the head chips, and thus, the resulting image pattern may appear distorted and different from the intended original image.